Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Multi-Carrier Spread Spectrum 49narrowband sub-channels. Systems with broadband sub-channels typically apply only fewnumbers of sub-channels, where each sub-channel can be considered as a classical DS-CDMA system with reduced data rate and ISI, depending on the number of parallelDS-CDMA systems. MC-DS-CDMA systems with narrowband sub-channels typicallyuse high numbers of sub-carriers and can be efficiently realized by using the OFDMoperation. Since each sub-channel is narrowband and spreading is performed in the timedirection, these schemes can only achieve a time diversity gain if no additional measuressuch as coding or interleaving are applied.Both multi-carrier spread spectrum concepts are described in detail in Chapter 2.1.4.2 Advantages and DrawbacksIn Table 1-16, the main advantages and drawbacks of MC-CDMA and MC-DS-CDMAare summarized. A first conclusion from this table can be derived:– The high spectral efficiency and the low receiver complexity of MC-CDMA makesit a good candidate for the downlink of a cellular system.– The low peak to average power ratio (PAPR) property of MC-DS-CDMA with lownumbers of subcarriers makes it more appropriate for the uplink of a multi-usersystem.1.4.3 Examples of Future Application AreasMulti-carrier spread spectrum concepts have been developed for a wide variety of applications.Table 1-16Advantages and drawbacks of MC-CDMA and MC-DS-CDMAMC-CDMAMC-DS-CDMAAdvantages Disadvantages Advantages Disadvantages– Simpleimplementationwith HadamardTransform andFFT– Low complexreceivers– High spectralefficiency– High frequencydiversity gaindue to spreadingin the frequencydirection– High PAPR,especially in theuplink– Synchronoustransmission– Low PAPR inthe uplink– High timediversity gaindue to spreadingin the timedirection– ISI and/or ICIcan occur,resulting inmore complexreceivers– Less spectralefficient if othermulti-carriermodulationschemes thanOFDM are used